The GIF89 transparent color extension allows an image to specify a transparent color, which can then be displayed as the same color as the background on which the image is to be displayed.
Support for viewing such images was first available in Mosaic 2.0 . Version 2.1 fixed a bug ... but it seems to be somewhat useable with Mosaic 2.0 anyway.
If that statement didn't make any sense, you can see here (if you have a compatible WWW browser) what I meant: the following picture has no background; the area around the flower is part of the GIF but is the same color as the rest of your display. It's not just a good match to the default Mosaic colors, I swear. :-) I use a strange purple Mosaic color-scheme myself .... and this looks great there.
There are several different methods for creating GIFs with transparent colors. If you don't know much about writing or compiling programs, then you will probably find TransWeb easiest to use.
If you only have access to an MS-DOS system, then you may want to skip the rest of this page and just read my instructions for using Transgif with MS-DOS.
If you only have access to a Macintosh running MacOS, then you may want to skip the rest of this page and just read about Transparency for the Macintosh.
However, TransWeb uses our web server to perform all its computations, and the transparentizing process is somewhat computationally expensive, so this method may be really slow and will increase the load of our already heavily loaded web server.
If you have perl on your system, the second method is the most flexible. If not, then you can try the third method.
This is the easiest way to transparentize images -- it requires absolutely nothing except that the image you wish to be transparentized be available on the WWW via a publicly-accessible URL.
However, TransWeb uses our web server to perform all its computations, and the transparentizing process is somewhat computationally expensive, so this method may be really slow and will increase the load of our already heavily loaded web server.
TransWeb is a service available from the MIT SIPB web server.
This is the simplest way to make transparent images -- it's written in perl, and requires no compiling to use (if perl is installed on your system).
I wrote some instructions for getting and using transgif.
There is a package of image manipulation utilities distributed under the name "netpbm". It's compileable for many platforms, perhaps even DOS, though I've never tried compiling it for anything other than various Unixes. This package is fairly flexible, and is most useful when used as are many unix "productivity tools" -- using a combination of several smaller programs to get the job, rather than using a single specific program.
I wrote some instructions for getting and using the netpbm utilities.
Please feel free to let me know what you think.
-Eric Mumpower (nocturne@mit.edu)